(1) Field of the Invention
The invention generally relates to a method and apparatus used in semiconductor manufacturing and, more particularly, to a valve used to isolate individual chambers in vacuum pressure cluster tools in the fabrication of integrated circuits.
(2) Description of Prior Art
Multiple chambered cluster tools have become generally accepted as an efficient means to process semiconductor wafers. The cluster tools allow several steps to be performed on a wafer without the necessity of moving the wafers into and out of a vacuum environment. Wafers are moved between a plurality of process chambers via a central transfer chamber within the vacuum environment. Each process chamber is isolated from the transfer chamber by means of a slit valve. The slit valve door is opened while the wafer is moved between the transfer and process chambers and closed during wafer processing.
Referring now to FIG. 1 depicting schematically a top view of a typical cluster tool, the Endura (5500) system. Robot wafer handlers 12, housed within the transfer chamber 10 and buffer chamber 11, allow wafers 14 to be rotated about the center of each chamber and moved radially into one of a plurality of chambers 16a-16j through slit valves 18a-18l. 
The slit valves 18 currently in use employ high pneumatic pressure to seal the wafer ports and isolate the process chambers 16 from the transfer chamber 10 or buffer chamber 11. The door of the slit valve 18 is typically composed of aluminum, while the seat for the door is typically composed of stainless steel. The high pressure used to close and seal the door results in contact between the door and seat thereby generating both aluminum and stainless steel particles. These particles may land upon the wafers being processed causing a reduction in the yield of the integrated circuits (ICs) being produced. Wear on the door and seat results in the need for their replacement. This replacement requires that the cluster tool be out of service for more than twelve hours. In addition, this high-pressure closure causes premature failure of an o-ring used on the slit valve 18. This o-ring is typically replaced as part of a preventative maintenance procedure based upon known o-ring life.
Other approaches for improving wafer-handling equipment exist. U.S. Pat. No. 4,692,115 to Aldridge et al. teaches a system for loading and unloading wafer-carrying boats into a thermal furnace that allows a soft wafer landing thereby reducing the particles generated. This system includes a door used to seal the furnace employing a multi-step opening procedure. U.S. Pat. No. 5,054,988 to Shiraiwa teaches a method for transferring wafers between cassettes and boats where dust created by the transfer mechanism is minimized. U.S. Pat. No. 6,042,623 to Edwards and U.S. Pat. No. 6,071,055 to Tepman describe systems for moving wafers into and out of a vacuum environment where throughputs are improved and pump-down times are reduced.
A principal object of the present invention is to provide a method that reduces the number of particles generated when closing the slit valve door.
Another object of the present invention is to provide a method that reduces the number of particles generated when closing the slit valve door and thereby improves the yield of finished integrated circuit (IC) die.
Another object of the present invention is to provide a method that reduces downtime for both scheduled and unscheduled maintenance on the slit valve door and seat.
Another object of the present invention is to provide a method that reduces consumable part usage by decreasing wear on the slit valve door and seat.
Another object of the present invention is to provide a method that improves the IC yield by reducing the generation of particles caused by the closing of the slit valve door and thereby requires fewer wafers in a test lot.
These objects are achieved using a system designed to control the slit valve door actuator pneumatic pressure and movement during the closing sequence thereby resulting in a soft touch landing. During the closing sequence, the slit valve door will begin closing at full pressure. When the door is nearly closed, a speed change sensor activates a circuit designed to reduce the closing pressure resulting in a soft touch landing. Once the door reaches the closed position, the full pressure is then reapplied to provide a proper vacuum seal for the chamber. Full pressure is applied during the door opening sequence.